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iron106


Mar 30, 2009, 8:35 AM
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Another stupid Idea about gear placements and impact force
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I have been climbing for quite some time now and have learned a lot over the years. I have 2 jobs and 2 kids now and spend more time thinking about climbing rather than actually climbing (other than the gym). Since many of you get the opportunity to spend a lot of time climbing I wonder what you all thought of my stupid idea.

With soft catches, impact force, rope stretch, extra gear placements, etc always being talked about, I have been thinking of other ways to reduce the amount of force on your gear placements. I wondered if you took a 2’ or 4’ nylon sling and tied an overhand knot or a figure 9 in it, if you would lower the amount of impact force on your gear, the same way your tie in knot absorbs your impact. The idea being that it is an improvised screamer sort of. I do think that it may kind of turn your “trad draw” into a “sport draw” a little. However with draws this length it may not matter (of course it depends on the situation).

It my just be another stupid idea. I am looking for your input.


Partner cracklover


Mar 30, 2009, 8:40 AM
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It certainly would lower the impact force, but only by a small amount. Of course, the more winds the knot had in it (the more sliding it would do) the more potential force it might be able to absorb. Take that to absurd lengths, and you might get close to a screamer.

GO


iron106


Mar 30, 2009, 8:43 AM
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Re: [cracklover] Another stupid Idea about gear placements and impact force [In reply to]
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Than is why I think the Figure 9 would be good however I am not sure of the reduction of the strength of the material. Also the Figure 9 is easier to untie after loading.


iron106


Apr 1, 2009, 6:01 AM
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I actually think the figure 8 is better than the figure 9. It will have more equal loading of the sling.


patto


Apr 1, 2009, 6:44 AM
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Surely this is a joke!

A tied sling has ~3cm to dissapate energy. A dynamic climbing rope usually has ~15m. So even if we assume that your knot is just as good at absorbing energy than your rope we are still talking about only one 500th of the energy absorbing capabilties a dynamic rope.

Forget trying to reinvent the wheel. We have perfectly good climbing ropes to reduce the forces in falls.


kachoong


Apr 1, 2009, 6:46 AM
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Is the gain in reducing impact force worth the reduction in overall strength from the knot(s)?

Why not replace the nylon sling with dynamic rope instead?


patto


Apr 1, 2009, 6:58 AM
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Re: [cracklover] Another stupid Idea about gear placements and impact force [In reply to]
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cracklover wrote:
It certainly would lower the impact force, but only by a small amount. Of course, the more winds the knot had in it (the more sliding it would do) the more potential force it might be able to absorb. Take that to absurd lengths, and you might get close to a screamer.

GO

No a screamer is fundamentally different. It has a much higher activation force and has a flat strain curve. This allows it and the rope to absorb more energy AND it limits the force.


iron106


Apr 1, 2009, 7:42 AM
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I'll have to double check however, I believe that it has been shown that even the nylon does reduce the impact on the anchor vs dyneema becuase of stretch in the material. This would be merely an improvised method of reducing the impact on smaller gear placements, crucial pieces, and marginal placements.

Also the strength of the material would not be a factor, because going from 22KN to 17.6KN is still much stronger than most gear.

In addition the tightening of the figre 8 knot, which would take up about a foot of material allowing it to stretch a lot under a heavy load.


anthonypmason


Apr 1, 2009, 8:16 AM
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You my friend need to step away and go climbing, stop over analyzing the wheel it aint broken, if you talking about marginal gear placements/ death falls then use a frigging screamer on that piece, knots do fail, there are many charts that show the %%, less knots the better.
Nylon does stretch a bit more than dyneema, but 1cm does not make any difference, it is the entire system that helps reduce the forces in a climbing fall.

If what you want is stretch use some bungee, like the ones used by bungee jumpers. fat 4" bungee cord woo-woo it is less likely to fail, unlike you're knots.


iron106


Apr 1, 2009, 8:26 AM
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anthonypmason wrote:
You my friend need to step away and go climbing, stop over analyzing the wheel it aint broken, if you talking about marginal gear placements/ death falls then use a frigging screamer on that piece, knots do fail, there are many charts that show the %%, less knots the better.
Nylon does stretch a bit more than dyneema, but 1cm does not make any difference, it is the entire system that helps reduce the forces in a climbing fall.

If what you want is stretch use some bungee, like the ones used by bungee jumpers. fat 4" bungee cord woo-woo it is less likely to fail, unlike you're knots.

You are right that I need to go climbing. In fact I am starting too feel sick, sick enough to go out and do some routes.

I do not know much about those bungee's though.

I do not think a figure 8 on a sling would fail.


antiqued


Apr 1, 2009, 8:43 AM
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The amount of energy absorbed by the tightening knot is the product of the force * displacement. In normal knot tightening, I would be surprised if you could get much more than an inch of slack between body weight and 2000lbf. Whatever you take out of a loosely tied knot with light forces (less than body weight) doesn't slow you down. (You accelerate more slowly, but you have more time to accelerate while the knot slips.)

So you would probably be better off with a softer rope, or fewer ham sammiches, than a regular overhand or 8.

The offset overhand and offset 8 will roll and consume a lot of rope, but the performance seems to be fairly unpredictable (http://www.xmission.com/...yer/testing/EDK.html. Since commercial screamers are pretty reproducible, they seem a better bet if you are really worried.


cchas


Apr 1, 2009, 8:54 AM
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I wouldn't worry about it and just go climbing using normal techniques. There is a far greateer chance that you will screw up then the gear failing. I have a friend that is 1855lbs and on a fairly regular basis I've belayed him on pretty good whippers on small gear (stuff smaller then blue aliens).

If you are concerned as said before, cut out those ham sammiches.


acorneau


Apr 1, 2009, 8:58 AM
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cchas wrote:
I have a friend that is 1855lbs ...

Damn, that dude is already at 8kN just hanging on the rope! You must have one bad-ass grip strength!



Tongue


Partner cracklover


Apr 1, 2009, 8:59 AM
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iron106 wrote:
I do not think a figure 8 on a sling would fail.

Nope, it wouldn't. However it would also be totally ineffective. At worst, it would give you a false sense of confidence in gear that a saner mind would know not to trust.

GO


dingus


Apr 1, 2009, 9:34 AM
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This would not produce any field measurable results.

And the rope does the energy absorbtion. The screamer activation buys the rope more time to absorb energy. The screamer itself absorbs none or very little.

What's wrong with buying a screamer or two?

DMT


iron106


Apr 1, 2009, 11:00 AM
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dingus wrote:
This would not produce any field measurable results.

And the rope does the energy absorbtion. The screamer activation buys the rope more time to absorb energy. The screamer itself absorbs none or very little.

What's wrong with buying a screamer or two?

DMT

Nothing is wrong with buying a screamer or two. However, if I do not have one on hand or used it, maybe something of this sort would work.

As the knot tightens it may absorb a small amount of impact force. Also I forget the source but there is a measurable difference between a rope that the knot has been loaded vs a tied and dressed knot. Granted the eight on a climbing rope is significantly larger than an eight on a sling.

I will somehow have to come up with some kind of test to figure this out.

Most likely you are all right in that it may not produce any meaningful results. Thus it is a stupid idea.


Partner angry


Apr 1, 2009, 11:04 AM
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I think there's a big difference between stupid idea and idea that won't work.

You're not being stupid but that fig-8 isn't going to be what saves your ass.


hafilax


Apr 1, 2009, 11:58 AM
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-Tie a loose knot in a sling.
-Measure the length with a light force stretching it.
-Measure the length after pulling it really hard.

If the difference in length is much less than the stretch in the rope then it won't have much of an effect.

The knot cinching effect only really lowers the impact force at high fall factors. It's like adding a meter of rope (or something like that). If you're already 20m out then it will have a small effect. If you're 1m out it will have a significant effect.

A screamer does work (in the physics sense). It applies a nearly constant force over the length of its stitches.


iron106


Apr 1, 2009, 12:43 PM
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I did find the following;
If an object of mass m=100kg (my weight)is dropped from height
h = 5m, then the velocity just before impact is
v = 9.89m/s. The kinetic energy just before impact is equal to
K.E. = 4900J.

If in addition, we know that the distance traveled after impact is
d =1m, then the impact force may be calculated using the work-energy principle to be

Average impact force = F = 4900N or 4.9 KN

If we change the distance slightly
d =1.05m, then the impact force may be calculated using the work-energy principle to be

Average impact force = F =4666 N. 4.6KN

That is adding 5 cm to the distance of the fall.

Are the assumptions correct?
Is this significant?
Maybe?


dingus


Apr 1, 2009, 12:50 PM
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yes the assumption that a loose knott is equivalent to an extra yard of slack... I don't believe this has any basis whatsoever.

Now if you care to visit the Yates website I think they have some specific analysis of what a screamer (read - load limiter) does in a fall. A knot bears no resemblance to a load limiter, at all. A knot is NOT (hehe) a load limiter.

DMT


hafilax


Apr 1, 2009, 12:58 PM
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The average force isn't as important as the peak force since the peak force is what breaks things. You want to know what the force as a function of time looks like. If the force during the cinching of the knot is small over the distance it tightens it will do nothing to reduce the peak impact force.

There isn't really a very good way at getting at this without experiment. The forces involved in slowing a climber are rather non-linear. My gut is telling me that knots in a sling will do very little to change the peak impact force even for falls approaching factor 2.

I would bet that making a long sling out of an old double or twin rope would work better than tieing knots in static material.


Partner rgold


Apr 1, 2009, 10:52 PM
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Well.

The assumption,

The amount of energy absorbed by the tightening knot is the product of the force * displacement,

serves as the basis for the following comments:

A tied sling has ~3cm to dissipate energy. A dynamic climbing rope usually has ~15m. So even if we assume that your knot is just as good at absorbing energy than your rope we are still talking about only one 500th of the energy absorbing capabilities a dynamic rope.

-Tie a loose knot in a sling.
-Measure the length with a light force stretching it.
-Measure the length after pulling it really hard.

If the difference in length is much less than the stretch in the rope then it won't have much of an effect.


I don't think the assumption is accurate, and consequently the comments based on it are also invalid. The assumption requires that the amount of force exerted during the knot tightening is constant, and this is certainly false---the force needed to extend goes up as the knot tightens. If the tension in the knotted strand is, say, proportional to its lengthening, then you essentially have a spring, perhaps with a fairly high spring constant, and the energy absorbed is proportional to the square of the elongation. But who knows, perhaps the tension is proportional to the square of elongation, or something even more "powerful."

This would not produce any field measurable results.

My intuition (as well as the OP's, it seems) agrees with this...but I wouldn't be quite so certain.

And the rope does the energy absorbtion. The screamer activation buys the rope more time to absorb energy. The screamer itself absorbs none or very little.

I don't think there is any validity to the "buying time" concept of screamer function, even though Yates advanced that theory when he couldn't explain his results ( that were never actually communicated) any other way. The screamer absorbs energy by forcing work to be done against the stitch resistance, and that's it. It is true that this does not amount to a whole lot of energy.

Now if you care to visit the Yates website I think they have some specific analysis of what a screamer (read - load limiter) does in a fall.

The numbers on the Yates site are a fantasy as far as I can tell. Nothing close to them has been observed when someone (other than Yates, perhaps) tests a screamer, and no theoretical calculation I know of predicts the force reductions claimed there. The fact that Yates relates load reduction to fall factors is prima facie evidence of a fundamental problem, since screamers, unlike the climbing rope, are not "scalable" and can at best absorb a fixed and relatively small amount of fall energy.

In Measurement of Dynamic Rope System Stiffness in a Sequential Failure for Lead Climbing Falls, Beverly and Attaway found that the tightening of the figure-8 tie-in knot during a fall soaked up a surprising amount of fall energy, absorbing the equivalent of nearly five feet of rope. This is a "field-measurable result," not a theoretical supposition. Whether anything remotely similar would be observed in flat static sling material is anyone's guess, but the result should perhaps keep us all suitably humble about what is and is not obvious in the realm of energy absorbtion.


patto


Apr 1, 2009, 11:53 PM
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rgold wrote:
Well.

The assumption,

The amount of energy absorbed by the tightening knot is the product of the force * displacement,

I don't think the assumption is accurate

Well you the laws of physics would be inclined to disagree with you. Work=force*displacement you can't escape that equation. Of course if the force is not canstant then it becomes an integral, but surely that is a given regardless.

rgold wrote:
The assumption requires that the amount of force exerted during the knot tightening is constant, and this is certainly false---
No the assumption does not require that.

But you are right in that if the knot has a higher spring constant it will absorb more energy in displacement. Though I doubt they even close to an order of magnitude different, dynamic ropes don't extend much on arm tenstion, knots do cinch down.

rgold wrote:
I don't think there is any validity to the "buying time" concept of screamer function, even though Yates advanced that theory when he couldn't explain his results ( that were never actually communicated) any other way. The screamer absorbs energy by forcing work to be done against the stitch resistance, and that's it. It is true that this does not amount to a whole lot of energy.

Now if you care to visit the Yates website I think they have some specific analysis of what a screamer (read - load limiter) does in a fall.

The numbers on the Yates site are a fantasy as far as I can tell. Nothing close to them has been observed when someone (other than Yates, perhaps) tests a screamer, and no theoretical calculation I know of predicts the force reductions claimed there. The fact that Yates relates load reduction to fall factors is prima facie evidence of a fundamental problem, since screamers, unlike the climbing rope, are not "scalable" and can at best absorb a fixed and relatively small amount of fall energy.

You put forward a good argument. I wasn't aware that testing does not support Yates claims. Though I've always been awar of the scalable problems of screamers.


Partner rgold


Apr 2, 2009, 5:31 AM
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Re: [patto] Another stupid Idea about gear placements and impact force [In reply to]
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Patto, the arguments I mentioned assume the force is constant during tightening. Of course there is an integral involved in general. I have never encountered anyone who says work=force times displacement in a problem who is referring to that integral, but perhaps I misunderstood in this case.


dead_horse_flats


Apr 2, 2009, 7:10 AM
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anthonypmason wrote:
If what you want is stretch use some bungee, like the ones used by bungee jumpers. fat 4" bungee cord woo-woo it is less likely to fail, unlike you're knots.

Except that it will become a nice piece of artillery, aimed at your head, when the placement fails.

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